Various systems are known in the current state of the art for producing an image which is reflected off the front windshield of a vehicle, the image in general being one that conveys information to an occupant of the vehicle, typically the driver. Display of the image at eye level enables the driver to receive the information thereby conveyed without turning his head towards instruments or other means for displaying information, which are conventionally arranged for example in the fascia of the vehicle.
However, vehicle windshields consist of laminated sheets of glass and/or polymers, which produce multiple reflections and refractions. In other words, each time a light ray passes into the windshield from one interface between two layers of the latter and another, the light flux emitted by an eye level display projector undergoes both reflection and refraction, so that the observer will perceive an image which is multiplied into at least two apparitions of the same image, albeit at different strengths. This is the image multiplication effect caused by the windshield, in which the multiple images consist of the main projected beam and at least one copy thereof, displaced from the main projected beam in what may be called the direction of image multiplication.
In order to provide a remedy for these drawbacks, it has been proposed to use, locally on the windshield, semi-reflective layers or holographic reflectors which have a very high reflectivity for a given wavelength, so causing the other reflections to be negligible. Because the layers of the windshield are substantially transparent to natural light, they would in principle be required not to impair the direct vision of the driver.
However, the semi-reflective layer is visible because of the angular dependence of its reflective response. This disadvantage gives rise to visual discomfort for the driver. Moreover, for safety reasons, this local deformation is forbidden in some zones of the windshield by most motor vehicle regulations.